To get prepared for potential nuclear accidents, the Hong Kong Observatory has established an Accident Consequence Assessment System to simulate the movement and dispersion of radioactive materials released into the atmosphere and assess the impact of nuclear accident on Hong Kong's environment. The atmospheric dispersion model of the Accident Consequence Assessment System can be applied in near or long-distance simulation to evaluate the dispersion of other suspended particles in the atmosphere that are of public concern, such as dust, volcanic ash, smoke caused by forest fires, etc., allowing us to estimate the dispersion and deposition of different suspended particles in the atmosphere based on observation data and computer prediction models.

For near-field atmospheric dispersion simulation, the Observatory’s focus is mainly on the assessment of dispersion of radioactive materials and employs an atmospheric dispersion model called Risø Mesoscale PUFF (RIMPUFF). RIMPUFF is one of the Lagrangian mesoscale atmospheric dispersion models commonly used in meteorology. It can estimate the concentration of radioactive materials in the air and calculate the corresponding radiation dose. Figure 1 is an example of RIMPUFF radiation plume dispersion simulation.

For long-range atmospheric dispersion simulation, the Observatory employs the FLEXible PARTicle (FLEXPART) atmospheric dispersion model. This is a particle dispersion model that can simulate long-range atmospheric dispersion. Figure 2 is an example of the FLEXPART atmospheric particle dispersion simulation.

Atmospheric dispersion models can also be applied to different atmospheric environmental events, on the condition that the concerned suspended particulates can be transported by winds. The model can perform simulations based on the source term of the suspended particulates and its specific properties (e.g. transformation and decomposition) to assess the dispersion of the suspended particulates at different times and locations in the atmosphere. The following is an example of applying atmospheric dispersion models in atmospheric environmental assessment.

Figure 3(a) shows the dust storm dispersion simulation by FLEXPART in North China in early April 2023. Figure 3(b) shows the position of atmospheric dust observed at that time by the Himawari satellite of the Japan Meteorological Agency. Some atmospheric dust was covered by high clouds around the eastern China and the Yellow Sea. The simulation result shows resemblance with the actual situation as depicted in the satellite image. In short, atmospheric dispersion models are useful tools in assessing affected areas in various atmospheric environmental events.